This is broader than necessary. All that’s necessary for the argument is that mixing clay and one or more other soils can create an acidic soil — and that is already in the stimulus. We don’t need to know whether mixing any soil with an acidic soil will produce an acidic soil.

If mixing clay soil with acidic soil means that the soil is no longer clay, then azaleas cannot flourish in clay soil — because as soon as we add any acidic soil to allow azaleas to flourish, it’s no longer clay. (B) fixes this problem.

This is broader than necessary for the argument. All that’s necessary is that mixing clay with at least some type of soil creates an acidic soil. It doesn’t matter if clay can be mixed with 100% of acidic soils or only 1 — either way, we can make clay soil into an acidic soil.

The argument concludes that azaleas can flourish in clay soil, because they can flourish in acidic soil and clay soil can be turned into an acidic soil. The argument doesn’t discuss other types of plants — whether tulips can flourish in acidic soil wouldn’t impact the argument.

In the context of this argument, we only care about whether the azaleas can flourish. We’re not concerned with whether they flourish slightly more in naturally acidic soils — as long as they can flourish in both soils, the argument is true.